Copper foil for a printed circuit board, a process and an apparatus for producing the same

ABSTRACT

This invention provides a process for producing an electrolytic copper foil for a printed wiring board by supplying current between a rotary cathode and an electrolytic anode in a copper electrolyte so as to electrodeposit copper on the surface of the rotary cathode, wherein an anode for high electric current is placed opposite to the electrodeposition starting surface of the rotary cathode in such a manner that a part of the anode is projected above a liquid level of the copper electrolyte, and the copper electrolyte existing between the anode for high electric current and the opposed rotary cathode surface is electrolyzed by providing a high electric current with a current density higher than that of the electrolytic anode, and an apparatus for producing an electrolytic copper foil for a printed wiring board having excellent properties.

This is a divisional of application Ser. No. 08/741,709, filed Oct. 31,1996, now U.S. Pat. No. 5,833,819.

FIELD OF THE INVENTION

This invention relates to a process for producing a copper foil for aprinted wiring board, which copper foil is substantially free from curlsand pinholes and is excellent in physical properties, and to anapparatus for producing such copper foil.

BACKGROUND OF THE PRIOR ART

There have heretofore been described processes for producing apinhole-free electrolytic copper foil for printed wiring boards inJapanese Patent Application Publication Gazette No. Hei 3-1391 (or1391/1991) and Japanese Patent Application Laid-Open Gazette No. Hei1-198495 (or 198495/1989).

However, an electrolytic cell having an electrolyte containing a certainconcentration of copper ions, a cathode surface which moves whileimmersing the surface thereof into said electrolyte, and an anodesurface installed to a position opposite to said cathode surface hasbeen used in the art of producing a copper foil according to JapanesePatent Application Publication Gazette No. Hei 3-1391 (or 1391/1991). Ina first zone through which the cathode surface passes in theelectrolytic cell, copper nucleuses are formed on the surface of thecathode by applying a pulsed first current density pulsating with valuesgreater and smaller than that of a limiting current density of copperion. Subsequently, in a second zone through which the cathode passes inthe electrolytic cell, a relatively smooth deposit of the copper foil isformed on the surface of the cathode by applying a current densitysmaller than the density of the limiting current density. Further, in athird zone through which the cathode passes in the electrolytic cell, aplurality of nodules are formed on the deposit of the copper foil byapplying a pulsed second high current density pulsating with valuesgreater and smaller than that of the limiting current density. The priorart according to Japanese Patent Application Publication Gazette No. Hei3-1391 (or 1391/1991) is intended to produce a surface treated copperfoil by giving a surface treatment including the process steps as statedabove.

More specifically, the art of producing a copper foil according toJapanese Patent Application Publication Gazette No. Hei 3-1391 (or1391/1991) is intended to form a highly pore free ultra-thin copper foilhaving an adhesive nodularized outer surface. Since a layer having thenodules is formed on an electroplated metal, however, there is providedat least one zone having a current density greater than the limitingcurrent density in an electrolytic cell. This current density zone isformed by a processing anode provided via a gap or an insulatingmaterial completely separated from a primary anode and is provided atthe outlet or inlet and the outlet of an electrolytic cell.

In the art of producing a copper foil according to Japanese PatentApplication Publication Gazette No. Hei 3-1391 (or 1391/1991), however,the copper foil has been so arranged that the first anode is placed inthe electrolytic cell and is set lower than the liquid level therein butthe process anode is not located in a position opposite to theelectrodeposition starting zone of the cathode surface, that is,opposite to the cathode surface in the vicinity of a surface of anelectrolyte. The current density is lower than that in the cathodesurface situated opposite to the first anode and a sufficiently highcurrent density is not obtainable there. Therefore, the copper foilobtained are such that a number of nucleuses are not formable initiallyand satisfactorily. As a result, the aforementioned art has notsucceeded in solving problems to be solved by this invention intended toprovide a copper foil substantially free from curls and pinholes.

The art disclosed in Japanese Patent Application Laid-Open Gazette No.Hei 1-198495 (or 198495/1989) is intended to carry out electrolysis withan electrolyte not containing gas at the initial and final stages ofelectrodeposition in order to obtain a pore-free copper foil bydischarging the spent electrolyte containing a large quantity of gasgenerated by the electrolysis from a submerged liquid outlet provided inthe upper portion of an electrolytic cell. Since the anode is placedunder the liquid level even in this case, the art disclosed therein,like Japanese Patent Application Publication Gazette No. Hei 3-1391 (or1391/1991), has not also succeeded in solving problems to be solved bythis invention.

The copper foil produced by the aforementioned prior art methods hasinternal strain and pinholes to varying degrees and the aforementionedprior art aimed at solving these problems still fails to attain itsobject.

There has been a recent tendency to make a copper foil thinner for usein printed wiring boards and has been developed a demand for a copperfoil free from internal distortion and pinholes. The internal distortionof a copper foil in particular develops as a curl phenomenon, which isrecognized from the fact that the edges portion of a copper foil turnsup when it is placed on a flat table, for example. The number ofpinholes and curls of a copper foil for printed wiring boards tends toincrease as its thickness is decreased and this has posed a seriousproblem as the demand for a thinner copper foil is increased.

When copper foil lamination is automatically conducted by means ofrobots, curls of the copper foil produced by the prior art methods tendto make the robot commit an error in handling the copper foil forprinted wiring boards, that is, make the robot fail to take hold of it;the problem is that the production of printed wiring boards is notsmoothly carried out. Therefore, a copper foil substantially free fromcurls has been desired.

SUMMARY OF THE INVENTION

An object of this invention is to provide a process for producing anelectrolytic copper foil for a printed wiring board, which copper foilis excellent in physical properties, that is it is substantially freefrom curls and pinholes, and an apparatus for producing such copperfoil.

The present inventors made intensive studies in attempts to solve theabove problems concerning the prior art and, as the result of theirstudies, they found out that the foregoing problems are solved bysetting an anode projected above the surface of an electrolyte flowingout by overflow, the anode being for use in making a high electriccurrent flow toward the electrodeposition starting surface of a rotarycathode separated from an electrolytic anode, and thereby attaining asupplement of a high electric current onto the surface of theelectrolyte, particularly in the vicinity of a vapor-liquid boundary.The present invention has thus been completed.

More specifically, this invention resides in a process for producing anelectrolytic copper foil for a printed wiring board by supplying currentbetween a rotary cathode and an electrolytic anode in a copperelectrolyte so as to electrodeposit copper on the surface of the rotarycathode, wherein an anode for high electric current is placed oppositeto the electrodeposition starting surface of the rotary cathode in sucha manner that a part of the anode for high electric current is projectedabove the liquid level of the copper electrolyte, and the copperelectrolyte present between the anode for high electric current and theopposite rotary cathode is electrolyzed by providing a high electriccurrent zone through which a high electric current with a currentdensity higher than that of the electrolytic anode is made to flow.

Another object of this invention resides in providing a copper foilsubstantially free from curls and pinholes, obtainable through the aboveprocess for producing the copper foil for a printed wiring board havingexcellent physical properties, and providing an apparatus for producingsuch copper foil.

Hereinbelow, this invention will be explained in more detail withreference to accompanying drawings.

FIG. 1 is a graph showing variations in current density from nucleationin the initial stage of electrodeposition up to crystal growth. In FIG.1, curve (a) represents variation in current density in an ideal case;curve (b), measures values in the case of Example 3 of this invention;curve (c), measures values in reference to Comparative Example 1; andcurve (d), measures values in reference to Comparative Example 2.

FIG. 2 is an enlarged fragmentary view showing the vicinity of anelectrolyte inlet in the case of Comparative Example 2.

FIG. 3 is an enlarged fragmentary view showing the vicinity of anelectrolyte inlet in the case of Example 2 of this invention.

FIG. 4 is a cross sectional view illustrating an apparatus formanufacturing a copper foil generally in use.

FIG. 5 is a cross sectional view illustrating an apparatus formanufacturing a copper foil according to this invention.

In FIGS. 2-5, reference numeral 1 denotes a rotary cathode; numeral 2designates, an electrolytic anode installed opposite to the rotarycathode; numeral 3 designates, an anode for high electric current inwhich the anode has a hole allowing an electrolyte to pass through, thehole being formed with a net or a comb or in any other form; numeral 3'designates, a conventional plate-like anode for high electric current;numeral 4 designates, an insulating plate for insulating the anode 3 forhigh electric current from the electrolytic anode 2; numeral 5designates, a take-up reel; and numeral 6 designates, a cell,respectively.

The process for producing a copper foil for a printed wiring boardaccording to this invention has two features. As shown in FIGS. 3 and 5one of the features is that the anode 3 for making a high electriccurrent flow toward the electrodeposition starting side is configuredlike a net, a comb or the like instead of a plate which hasconventionally been employed as shown in FIG. 2, so that an electrolyteis allowed to go in and out freely through the anode. The other featureof this invention is that a number of crystal nucleuses is formed on theelectrodeposition starting surface by letting the high electric currentflow between the anode for the high electric current and the cathodesurface opposite to it, the high electric current having a currentdensity higher than the current density between the electrolytic anodeand the cathode surface opposite to it.

That is to say, according to the conventional electrolytic method, asshown in FIG. 2, the anode 3' for use in initial electrodeposition iscompletely submerged in the electrolyte and while the electrolyte iscaused to pass over the anode, it is attempted to effect nucleation bymeans of the high electric current.

While trying to consider fault with the prior art, the present inventorshave discovered the fact that nucleation is completed in an extremelyshort time in the initial stage of electrolysis. As is obvious from FIG.1, the time required ranges from 0.1 to 1 sec (the time required forpassage across the high electric current zone) and the present inventorshave found the that the current density at the moment theelectrodeposition starts is the most important factor.

In a case where a high electric current is made to flow through thesubmerged anode for initial electrodeposition, as shown in FIG. 2,according to the electrolytic method of the prior art, the currentdensity at the moment the electrodeposition starts becomes lower thanthe average current density of said anode. Therefore, satisfactorynucleation cannot be accomplished, as shown by the curve (d) of FIG. 1.In the process according to this invention, on the other hand, thepresence of the anode 3 placed opposite to the cathode surface of theelectrodeposition starting zone as shown in FIGS. 3 and 5 allows toapply the sufficient current density at the moment the electrodepositionstarts as it is obvious from the curve (b) of FIG. 1.

In addition to this, it is ideal if the current variation until anelectrodeposition starting surface on a cathode arrives to the placeopposite to the ordinary electrolytic anode 2 through the anode 3 forhigh electric current after the electrodeposition starting surface ofthe cathode runs into the electrolyte, follows the curve (a) of FIG. 1and in the case of this invention, the current variation follows thecurve (b) and this is proved to be substantially ideal in comparisonwith the curve (d) in the case of the prior art method.

The anode 3 for high electric current used in the process of producing acopper foil for a printed wiring board according to this invention canbe installed by suspending said anode 3 like a net such as a lath DSEmade by Permeleck Co., for example, at the inlet of the ordinaryelectrolytic anode 2 or otherwise setting it on the stage of theinsulating plate 4. As far as the electrolyte is allowed to readily passthrough the anode 3 for high electric current, it is not restricted tosaid anode 3 like the net but may be prepared by boring a plurality ofholes of suitable dimensions in a comb or plate-like anode, for example.The anode 3 for high electric current like this is preferably such thatan electrolyte containing bubbles is allowed to readily pass through itin order to remove a large amount of gas generated in the vicinity ofthe anode due to electrolysis.

When the anode 3 for high electric current is installed, the followingprecaution ought to be taken, that is, the anode 3 has been immersed andsimultaneously a part of the anode remains to project from the surfaceof the electrolyte. If the anode 3 is arranged so that it is verticallymovable by making free use of various mechanisms and technology known inthe art, fluctuations in the liquid level of the electrolyte can simplybe dealt with even in a case where the quantity of electrolyte suppliedis varied.

What is the most important in the process of this invention is that ahigh electric current is supplied to the rotary cathode 1 from the anode3 for high electric current so as to make a sufficient current densityfor 0 to 1 sec. until nucleation is completed immediately afterelectrodeposition is started. The current ranges from 1.0 to 3.0 A/cm²and preferably 1.5 to 2.5 A/cm². In this case, nucleation will not becarried out satisfactorily at less than 1.0 A/cm², whereas a level inexcess of 3.0 cm² is undesirable because the deterioration of the anodeoccurs. And also, since a high current density is applied to the rotarycathode 1, nucleation is completed in a shorter time than preferabletime and crystal growth is started while the high current density ismaintained. As a result of this, granular copper deposition calledburned plating is brought about, which badly affects the physicalproperties of the copper foil obtained (see burned plating area of FIG.1).

In FIG. 1, the curve (a) represents the most ideal nucleation curve. Thehighest current density is applied immediately after electrodepositionis started and the current density lowers as nucleation proceeds. Whenthe nucleation is completed, the curve (a) converges to the ordinaryelectrolytic current density without entering the burned plating area.

In FIG. 1, the curve (b) represents variations in the current densityaccording to Example 2 of this invention and a curve of current densityvariations close to an ideal one.

In FIG. 1, the curve (d) represents variations in the current density inthe case of Comparative Example 2 which follows the conventional method,depicting insufficient current at the time of nucleation. The curve (d)shows the current density from the final stage of nucleation through theinitial stage of crystal growth which has entered the burned platingarea. In this case, the current density in the initial stage of crystalgrowth is to intrude deeply into the burned plating area, thus effectinga bad influence on the physical properties of the copper foil obtained.

In the process of producing a copper foil for a printed wiring boardaccording to this invention, the copper foil for a printed wiring boardis substantially free from curls and pinholes with a tensile strength ofnot less than 44.8 kg/mm² and an elongation of not less than 8.5% in anormal temperature, and an elevated temperature (measured value in atemperature of 180° C.) tensile strength of not less than 20.9 kg/mm²,an elongation of not less than 5.1% and a surface roughness Rmax of notgreater than 3 μm on the deposited side (matte side).

In the process of producing a copper foil according to this invention,it is possible to obtain the copper foil which is substantially freefrom curls and pinholes and has excellent physical properties by lettinga current having a high current density flow on a cathode at the momentan electrodeposition starting surface enters the electrolyte to cause anumber of high-density crystal nucleuses to be formed.

Moreover, an apparatus for producing an electrolytic copper foil for aprinted wiring board according to this invention, comprising a rotarycathode 1 and an electrolytic anode 2 installed opposite to the rotarycathode 1, wherein the copper foil for a printed wiring board ismanufactured by electrolyzing a copper electrolyte supplied between therotary cathode 1 and the electrolytic anode 2, and an anode 3 forletting flow a high electric current having a current density higherthan that of the electrolytic anode 2 toward the electrodepositionstarting surface of the cathode 1, provided on the electrolytic anode 2via an insulating plate 4 in such a manner that a part of the anode isprojected above the liquid level of the copper electrolyte.

Since the anode 3 for high electric current is provided on theelectrolytic anode 2 via the insulating plate 4 in such a manner that apart of the anode 3 is projected above the liquid level of the copperelectrolyte, electrolytic copper foil having excellent physicalproperties can be manufactured by setting the anode projected above thesurface of the electrolyte flowing out by overflow so as to supply ahigh electric current to the electrodeposition starting surface of thecathode, that is, to the cathode in the vicinity of a vapor-liquidboundary.

Effect of the Invention

The process of producing a copper foil for a printed wiring boardaccording to this invention is designed to make the copper foil freefrom curls and pinholes with simple electrolytic facilities and allowsthe facilities to be controlled freely. Therefore, the copper foilobtained thereby has not only excellent physical properties (hightensile strength, low roughness) but also elevated temperature-physicalproperties capable of satisfactorily preveating foil cracks which haveposed a serious problem in multilayered printed wiring boards usedmainly in recent years.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing variations in current density from nucleationat the initial stage of electrodeposition up to crystal growth.

FIG. 2 is an enlarged fragmentary view showing the vicinity of anelectrolyte inlet of Comparative Example 2.

FIG. 3 is an enlarged fragmentary view showing the vicinity of anelectrolyte inlet of Example 3 of this invention.

FIG. 4 is a cross sectional view illustrating an apparatus for producingan electrolytic copper foil generally in use.

FIG. 5 is a cross sectional view illustrating an apparatus for producingan electrolytic copper foil according to this invention.

FIG. 6 is a model diagram of crystal growth when nucleation is denselycarried out.

FIG. 7 is a model diagram of crystal growth when nucleation is coarselycarried out.

In the drawings, numeral 1 indicates a rotary cathode, numeral 2 anelectrolytic anode, numeral 3 an anode for high electric current,numeral 3' a conventional plate-like anode for high electric current,numeral 4 an insulating plate, numeral 5 a take-up reel and numeral 6 acell, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described concretely with reference to thefollowing Examples and Comparative Examples. Examples 1 through 3 andComparative Examples 1 through 3 are concerned with specifying anoptimum range of current densities with respect to anodes for highelectric current, whereas Examples 4 through 6 and Comparative Examples4 through 6 are concerned with specifying applied time of an anode forhigh electric current.

EXAMPLE 1

In an apparatus for continuously producing a copper foil by letting passan electrolyte containing copper ions between a cylindrical cathode 1which was kept rotating and an electrolytic anode 2 positioned oppositeto the cylindrical cathode 1 as shown in FIG. 4, a net-like anode 3 forhigh electric current was installed via an insulating plate 4 on theelectrolytic anode 2 in such a manner that the anode 3 for high electriccurrent projected above the surface of the overflowing electrolyte in aninlet (electrolysis starting) portion where the electrodepositionstarting surface of a cathode runs into in the electrolyte as shown inFIG. 3 (a height of the insulating plate: 2 mm, a height of the anode:50 mm, and a depth of immersing liquid: 10 mm). While a current of 1.1A/cm² was kept flowing through the anode 3, electrodeposition wascarried out under the following conditions to prepare copper foils 18 μmand 12 μm thick.

Copper ion concentration: 80 g/l,

Sulfuric acid concentration: 110 g/l,

Chloride ion concentration: 20 mg/l,

Liquid temperature: 50° C.,

Current density of electrolytic anode 2: 0.6 A/cm²,

Gelatin concentration: 3 ppm, and applied time of the anode 3 for highelectric current: 0.5 sec.

EXAMPLE 2

Electrodeposition was carried out under the same conditions as those inExample 1 except that the current density of the anode 3 for highelectric current was set at 1.5 A/cm² to prepare copper foils 18 μm and12 μm thick.

EXAMPLE 3

Electrodeposition was carried out under the same conditions as those inExample 1 except that the current density of the anode 3 for highelectric current was set at 2.5 A/cm² to prepare copper foils 18 μm and12 μm thick.

Comparative Example 1

Electrodeposition was carried out under the same conditions as those inExample 1 except that the current density of the anode 3 for highelectric current was set at 0.9 A/cm² to prepare copper foils 18 μm and12 μm thick.

The copper foil thus obtained exhibited no pinholes but a few curls.

Comparative Example 2

In the apparatus for continuously producing a copper foil by lettingpass an electrolyte containing copper ions between the rotatingcylindrical cathode 1 and the electrolytic anode 2 positioned oppositeto the cylindrical cathode 1 as shown in FIG. 4, a plate-like anode 3'(overflow type anode for high electric current) was installed at aninlet (electrolysis starting) portion (height of the insulating plate: 2mm, height of the anode: 10 mm). While a current of 1.5 A/cm² was keptflowing through the anode 3', electrodeposition was carried out underthe same conditions as those in Example 2 except that the anode 3 forhigh electric current in Example 2 was replaced with the anode 3' toprepare copper foils 18 μm and 12 μm thick.

The copper foils thus obtained exhibited both pinholes and curls.

Comparative Example 3

In the apparatus for continuously producing a copper foil by lettingpass an electrolyte containing copper ions between the rotatingcylindrical cathode 1 and the anode positioned opposite to thecylindrical cathode 1 as shown in FIG. 4. electrolysis was carried outunder the same conditions as those in Example 1 except that said anode 3for high electric current was not provided to prepare copper foils 18 μmand 12 μm thick.

EXAMPLE 4

Electrodeposition was carried out under the same conditions as those inExample 2 except that the applied time for electrolysis at the anode 3for high electric current was 0.1 sec to prepare copper foils 18 μm and12 μm thick.

EXAMPLE 5

Electrodeposition was carried out under the same conditions as those inExample 4 except that the applied time for electrolysis at the anode 3for high electric current was 0.5 sec to prepare copper foils 18 μm and12 μm thick. Although this example was carried out under entirely thesame conditions as those in Example 2, it was listed as Example 5 forconvenience of description.

EXAMPLE 6

Electrodeposition was carried out under the same conditions as those inExample 4 except that the applied time for electrolysis at the anode 3for high electric current was 1.0 sec to prepare copper foils 18 μm and12 μm thick.

Comparative Example 4

Electrodeposition was carried out under the same conditions as those inComparative Example 3 to prepare copper foils 18 μm and 12 μm thick.Although this Comparative Example was carried out under entirely thesame conditions as those in Comparative Example 3, it was listed asComparative Example 4 for convenience of description.

The copper foils thus obtained exhibited both pinholes and curls.

Comparative Example 5

Electrodeposition was carried out under the same conditions as those inExample 4 except that the applied time for electrolysis at the anode 3for high electric current was 0.05 sec to prepare copper foils 18 μm and12 μm thick.

The copper foils thus obtained exhibited both pinholes and curls.

Comparative Example 6

Electrodeposition was carried out under the same conditions as those inExample 4 except that the applied time for electrolysis at the anode 3for high electric current was 2.0 sec to prepare copper foils 18 μm and12 μm thick.

The copper foil thus obtained exhibited fragility and lowered utilitytogether with many pinholes though 0 mm curling.

TEST EXAMPLE 1

The copper foils prepared according to Examples 1-6 and ComparativeExamples 1-6 were subjected to pinhole test by Pinhole Evaluation DyePenetration Method defined in the IPC-TM-650 to examine the number ofpinholes per m².

Further, the copper foil prepared according to Examples 1-6 andComparative Examples 1-6 was cut into pieces of 10 cm-square as samplesand these samples were placed on a flat table with the cathode side(shiny side) downward to measure the raised height (curling) at fourcorners of each piece. The internal distortion of each sample wasexpressed by the mean value of curls of the four corners. The thusobtained test results are shown in Tables 1 and 3.

                  TABLE 1                                                         ______________________________________                                                 18 μm-thickness                                                                             12 μm-thickness                                             curl   pinhole     curl pinhole                                    Test No.   (mm)   (the number)                                                                              (mm) (the number)                               ______________________________________                                        Example 1  0      0           0    0                                          Example 2  0      0           0    0                                          Example 3  0      0           0    0                                          Comp. Ex. 1                                                                              3      0           5    0                                          Comp. Ex. 2                                                                              10     38          23   61                                         Comp. Ex. 3                                                                              18     55          30   110                                        ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                 18 μm-thickness                                                                         12 μm-thickness                                                 curl     pinhole   curl   pinhole                                  Test No.   (height) (the number)                                                                            (mm)   (the number)                             ______________________________________                                        Example 4  0        0         0      0                                        Example 5  0        0         0      0                                        Example 6  0        0         0      0                                        Comp. Ex. 4                                                                              18       55        30     110                                      Comp. Ex. 5                                                                              2        2         3      5                                        Comp. Ex. 6                                                                              0        320       0      870                                      ______________________________________                                    

TEST EXAMPLE 2

The roughness (Ra, Rz and Rmax) of the deposited side, tensile strengthand elongation of the copper foil prepared according to Examples 1-6 andComparative Examples 1-6 were measured at room temperature and elevatedtemperature (measured values in atmosphere of 180° C.). The thusobtained results were shown in Tables 2 and 4.

                  TABLE 2                                                         ______________________________________                                        (18 μm thickness)                                                                           Tensile strength                                                                           Elongation                                      Roughness of the (Kg/mm.sup.2)                                                                              (%)                                             deposited side (μm)                                                                         room     elevated                                                                              room elevated                               Test No.                                                                              Ra     Rmax   Rz   temp.  temp. temp.                                                                              temp.                            ______________________________________                                        Example 1                                                                             0.41   3.3    2.8  44.8   20.9  8.5  5.1                              Example 2                                                                             0.36   3.1    2.5  46.3   22.2  10.1 5.5                              Example 3                                                                             0.36   2.8    2.3  45.0   22.0  9.4  5.2                              Comp. Ex. 1                                                                           0.50   4.3    3.2  39.8   18.2  7.2  4.0                              Comp. Ex. 2                                                                           0.54   4.8    3.6  36.7   17.0  7.2  2.2                              Comp. Ex. 3                                                                           0.74   6.2    5.3  34.5   16.3  5.9  1.8                              ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        (18 μm thickness)                                                                           Tensile strength                                                                           Elongation                                      Roughness of the (Kg/mm.sup.2)                                                                              (%)                                             deposited side (μm)                                                                         room     elevated                                                                              room elevated                               Test No.                                                                              Ra     Rmax   Rz   temp.  temp. temp.                                                                              temp.                            ______________________________________                                        Example 4                                                                             0.39   3.2    2.7  45.9   21.2  9.3  5.1                              Example 5                                                                             0.36   3.1    2.5  46.3   22.2  10.1 5.5                              Example 6                                                                             0.38   3.2    2.6  46.0   22.3  10.0 5.6                              Comp. Ex. 4                                                                           0.74   6.2    5.3  34.5   16.3  5.9  1.8                              Comp. Ex. 5                                                                           0.51   4.2    3.2  41.6   19.7  8.0  4.6                              Comp. Ex. 6                                                                           1.07   8.2    6.7  24.1    8.9  0.8  0.5                              ______________________________________                                    

As set forth above, the influence of the initial electrodeposition inthe process of the electrolytic copper foil may be summarized as shownin Table 5.

In the electrolysis process according to this invention, the nucleationis densely carried out initially. As a result, the copper foil thusobtained is substantially free from curls and micropore, and also thesmoothness of deposited side (matte side) is improved. FIG. 6 is a modeldiagram of crystal growth when the nucleation is densely carried out.

When electrodeposition is conducted under the prior art methods,nucleation is coarsely carried out. The copper foil thus obtained haveconsiderable curls and micropore, thus making the roughness of a matteside greater. FIG. 7 is a model diagram of crystal growth when thenucleation is coarsely carried out.

                  TABLE 5                                                         ______________________________________                                        Relationship between Crystal Growth State Resulting from Inital               Electrodeposition and Curl/Pinhole                                            Initial              Physical properties obtained                             electro-                                                                              State of crystal growth       Roughness                               deposition                                                                            obtainable under the initial  on                                      state   electrodeposition state                                                                        Curl   Pinhole                                                                             matte side                              ______________________________________                                        Many    Growth with deposited side                                                                     Small  Few   Low                                     nucleuses                                                                             kept smoothess as many                                                are densely                                                                           nucleuses are formed to                                               formed  generate a dense initial                                                      electrodeposited layer and                                                    further uniform electro-                                                      deposition is conducted.                                              A few   Growth having coarse large                                                                     Large  Many  High                                    nucleuses                                                                             crystals and expanding                                                are coarsely                                                                          radially, vertically and hori-                                        formed  zontally, centering on initially                                              electrodeposited rough                                                        nucleuses.                                                            ______________________________________                                    

What is claimed is:
 1. A process of producing an electrolytic copperfoil for a printed wiring board comprising the steps of:(a) supplyingelectric current between a rotary cathode (1) and each of anelectrolytic anode (2) and an anode for high electric current (3) in acopper electrolyte, to electrodeposit copper on a surface of said rotarycathode; said copper electrolyte having a level; said rotary cathodehaving an electrodeposition starting surface; said anode for highelectric current having a hole or being shaped as a net or comb to allowthe copper electrolyte to go through in and out freely and being placedopposite to said electrodeposition starting surface of said rotarycathode in such a manner that a part of the anode (3) for high electriccurrent is projected above said liquid level of the copper electrolyte;and the electric curent provided in a high electric current zone, whichlies between said electrodeposition-starting surface of the rotarycathode and the anode for high electric current (3) has a higher currentdensity than the electric current provided between said rotary cathodeand the electrolytic anode (2) and (b) peeling an electrolytic copperfoil thus electrodeposited off the surface of said rotary cathode. 2.The process according to claim 1, wherein the higher current density ofthe electric current provided in said high electric current zone rangesfrom 1.0 to 3.0 A/cm².
 3. The process according to claim 1, wherein theperiod of time wherein a point on the surface of said rotary cathodepasses through said high electric current zone ranges from 0.1 to 1second per rotation of said rotary cathode.
 4. The process according toclaim 1, wherein said electrolytic copper foil produced in step b) issubstantially free from curls and pinholes and has a tensile strength ofnot less than 44.8 kg/mm² and an elongation of not less than 8.5% atnormal temperature.
 5. The process according to claim 1, wherein saidanode (3) is insulated from said anode (2) by means of an insulatingplate.